When inputting Interior Surfaces, you have the ability to indicate the Zone that the surface is Adjacent To as part of the inputs. Since different Zones can have different Floor Multipliers, the CBECC engine requires that the Zones on each side of the interior surface must have the same Floor Multiplier.
If you wish to change the Floor Multiplier, go to the Zone level of the Building Tree and click on the Building Story. From there you can edit the Floor Multiplier.
Another option is to edit the Interior Surface and change the Adjacent To setting to be another Zone that has the same Floor Multiplier, or you could even set the surface as Adjacent To itself.
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